1. Field of the Invention
The present invention relates to a method of manufacturing a color selecting mask.
2. Description of the Related Art
Color cathode-ray tubes have a color selecting mask positioned in confronting relation to a color phosphor screen for controlling scanning electron beams corresponding to respective colors to land on the patterns of color phosphors.
In general color cathode-ray tubes, the color selecting mask comprises a shadow mask having a plurality of circular beam passage holes each corresponding to a triplet of red, green, and blue phosphor dots on the color phosphor screen.
The shadow mask comprises a metallic sheet pressed to a slightly domed shape, and has its peripheral edges welded to and held by a frame. Since the shadow mask is held by the frame without being tensioned, the shadow mask suffers doming which causes the displayed colors to be shifted out of registry when the temperature of the shadow mask is increased by the scanning electron beams and the shadow mask is expanded by the heat. To avoid such a drawback, it is customary to construct the shadow mask of an expensive material having a low coefficient of expansion and increase the thickness of the shadow mask for greater mechanical strength.
A Trinitron (registered trademark) color cathode-ray tube has a color selecting mask in the form of an aperture grille or slot mask that confronts a color phosphor screen of the tube.
The color phosphor screen has an array of parallel vertical stripes (not shown) of red, green, and blue phosphors. The color selecting mask lying in facing relationship to the color phosphor screen has a plurality of parallel vertical slots or apertures for passing an electron beam therethrough, the slots extending along the vertical phosphor stripes on the color phosphor screen from the upper to lower edge of at least an entire effective screen area.
The slots are defined in a thin metal sheet of highly pure iron that has a thickness ranging from 0.08 to 0.15 mm. The thin metal sheet is supported on a centrally open frame.
The frame comprises a pair of confronting sides spaced from each other and a pair of confronting arms spaced from each other and extending between the sides. The sides have respective arcuate front end surfaces which are part of a cylindrical surface, and the thin metal sheet is placed on and extends between the sides.
The thin metal sheet is installed on the frame as follows: The sides are pulled toward each other by a turnbuckle, and then the thin metal sheet is fixed to the arcuate front end surfaces of the sides by welding its opposite edges near the ends of the slots. Thereafter, the turnbuckle is removed to release the frame of the forces that have been applied to the sides by the turnbuckle. Therefore, the web regions between the slots of the thin metal sheet are held under tension along the slots due to the tendency of the frame to recover its shape.
Recent demands for larger-size color cathode-ray tubes have resulted in longer web regions between the slots of the thin metal sheet. The longer web regions are, however, more liable to vibrate owing to voices and shocks applied when the electron beam passes through the aperture grille toward the color phosphor screen, shifting the displayed colors out of proper registration.
For suppressing the vibration of the web portions of the thin metal sheet, it has been customary practice to increase the thickness of the thin metal sheet for higher rigidity or increase the thickness of the frame for greater resilient recovery forces.
One process of defining the slots in the thin metal sheet which is relatively thick for vibration suppression is to etch the thin metal sheet on its opposite surfaces using photolithography.
Such an etching process will be described below with reference to FIGS. 1A through 1D of the accompanying drawings. First, as shown in FIG. 1A, an etching mask 11A of photoresist having a striped pattern of openings 1WA is formed on one surface 1A of a thin metal sheet 1 by a photolithographic process including photoresist coating, exposure, development, and photoresist removal. Thereafter, another etching mask 11B of photoresist having a striped pattern of openings 1WB is formed on the opposite surface 1B of the thin metal sheet 1 by photolithography. The openings 1WB are aligned with, and wider than, the respective openings 1WA.
Then, as shown in FIG. 1B, the thin metal sheet 1 is etched on both the surfaces 1A, 1B with an etching solution of ferric chloride (FeCl.sub.3), for example, to form grooves in the opposite surfaces 1A, 1B through the openings 1WA, 1WB in the etching masks 11A, 11B.
Thereafter, as shown in FIG. 1C, a protective film 12 of varnish, for example, is deposited in the grooves in the surface 1A. Using the protective film 12 as an etching mask, the thin metal sheet 1 is etched again on the surface 1B relatively gradually with an etching solution of FeCl.sub.3, for example, having a relatively low concentration until the protective film 12 is exposed in the grooves in the surface 1B, as shown in FIG. 1D.
Subsequently, the protective film 12 is removed. In this manner, a striped pattern of slots 2 for the passage of an electron beam therethrough are defined in the thin metal sheet 1. As shown in FIG. 2 of the accompanying drawings, the slots 2 extend longitudinally perpendicularly to the sheet of FIG. 2, and each have a cross-sectional shape of "8". Since the thin metal sheet 1 is etched twice, at a lower etching rate in the second etching step, the etching time can be controlled more easily and reliably than if the thin metal sheet 1 were etched once, even though the thin metal sheet 1 is relatively thick. Consequently, the thin metal sheet 1 is prevented from being excessively etched, and can be etched to a desired depth accurately. As a result, the effective width of each of the slots 2, i.e., the distance SW between opposite edges 5 formed in each of the slots 2 by the etching process, can be controlled highly accurately in the thin metal sheet 1 that is relatively thick. One problem with the above etching process is that the efficiency is lower than if the thin metal sheet 1 were etched once.
With the edges 5 formed, the slot 2 is defined by curved tapered surfaces 6 extending from the opposite surfaces 1A, 1B to the edges 5.
FIG. 3 of the accompanying drawings illustrate, in cross section, the manner in which an electron beam is applied through the color separating mask 4 to a color phosphor screen 10. In FIG. 3, an electron beam Ei travels through a slot 2 to the color phosphor screen 10 and hits the striped color phosphors for colored light emission. The color phosphor screen 10 produces an electron beam Er.sub.1 due to secondary electron emission, and the electron beam Er.sub.1 is reflected by the surface of the color separating mask 4 and a curved tapered surface 6, resulting in scattered electron beams Es and a reflected electron beam Er.sub.2. Consequently, the color phosphor screen 10 generate colors inaccurately, and the contrast and purity of the generated colors are lowered. If the slots 2 were formed in the color separating mask 4 in one etching step, however, the areas of the curved tapered surfaces 6 would further be increased, and the contrast and purity of the generated colors would further be lowered.
As described above, the color separating masks of the general Trinitron color cathode-ray tubes are composed of a thin metal sheet of relatively large thickness. However, inasmuch as the thin metal sheet of relatively large thickness is relatively heavy, the Trinitron color cathode-ray tubes are also relatively heavy.
The effective width SW (see FIG. 2) of each of the slots 2 is about 50% of the thickness t of the thin metal sheet 1 due to limitations posed by the etching process. Because the thin metal sheet 1 is of relatively large thickness, the width SW of each slot 2 is also relatively large in proportion to the thickness of the thin metal sheet 1. As a result, the slots 2 are not defined at a high density or in a fine pattern.
In an effort to solve the above problems, the applicant has proposed a color cathode-ray tube having a color selecting mask or aperture grille that is of improved accuracy, can be manufactured at an increased rate of production, has a reduced weight, and includes slots or apertures defined in a fine pattern.
As disclosed in Japanese laid-open patent publication No. 4-126341, the proposed color cathode-ray tube includes a color phosphor screen having parallel striped patterns of respective color phosphors and a color separating mask in the form of an aperture grille positioned in confronting relationship to the color phosphor screen and having a number of parallel slots or apertures extending along the parallel striped patterns of respective color phosphors for passing an electron beam therethrough. The slots are defined in a thin metal sheet which is mounted on a centrally open frame and kept taut under tension along the slots. The thin metal sheet has a small thickness of 0.05 mm or less.
Although the thickness of the thin metal sheet is small, it has been possible to suppress vibrations of the web portions thereof between the slots, which vibrations would otherwise be caused by voices and shocks applied thereto. The reasons for the suppressed vibrations are as follows:
If each of the web portions between the slots or apertures of the color selecting mask is regarded as a chord, then the resonant frequency f of the chord is given by the following equation: EQU f=(gt/.rho.).sup.1/2 /2Ls
where g is the gravitational acceleration, .rho. the linear density of the chord, T the stress developed in the chord, and Ls the length of the chord. Heretofore, if the length Ls of the chord increases as the size of the color cathode-ray tube increases, then the stress T is increased to increase the resonant frequency f away from a main range frequencies of vibrations caused by voices and shocks. According to the proposed color cathode-ray tube, the thickness of the thin metal sheet of the color selecting mask is reduced to reduce the linear density .rho. of the chord, so that the resonant frequency f is increased away from a main range of frequencies of vibrations caused by voices and shocks. Therefore, even though the thickness of the thin metal sheet is increased, the vibrations of the web portions of the thin metal sheet are suppressed. The proposed color cathode-ray tube can thus prevent produced colors from being brought out of registry owing to voice- and shock-induced vibrations, and can display high-quality colored images on the color phosphor screen.
FIG. 4 of the accompanying drawings shows, in cross section, a thin metal sheet 1 of a color selecting mask or aperture grill 4 of the proposed color cathode-ray tube. As shown in FIG. 4, the thin metal sheet 1 is so thin that slots or apertures 2 can accurately be defined in the thin metal sheet 1 in one etching step effected on one surface thereof. Since the time required to etch the thin metal sheet 1 is shortened, the rate of production is improved. In addition, the material of the thin metal sheet 1 is reduced, and the color selecting mask 4 can be manufactured with an increased yield.
Because the width SW of each of the slots 2 that are formed by etching is about 50% of the thickness t of the thin metal sheet 1, the width SW is relatively small as the thickness of the thin metal sheet 1 is small. The color selecting mask 4 is highly accurate in dimensions, and the slots 2 are defined at a high density or in a fine pattern.
As the thickness of the thin metal sheet 1 is small, the areas of curved tapered surfaces 6 of the holes 2 are also small. As shown in FIG. 5 of the accompanying drawings, therefore, any electron beams Es, Er.sub.2 reflected and scattered by the curved tapered surfaces 6 are suppressed. Consequently, any deterioration of the contrast and purity of colors produced by the color cathode-ray tube is minimized for displaying finely defined images on the color phosphor screen 10.
Reducing the thickness of the thin metal sheet 1 makes it possible to reduce the rigidity and weight of the frame that supports the thin metal sheet 4. The reduced frame weight is in turn effective to reduce the amount of electric energy that is required to be supplied to a degaussing coil that demagnetize an external magnetic field applied to the color cathode-ray tube. The electric power requirement of the color cathode-ray tube is lowered, and the color cathode-ray tube can reliably be demagnetized.
If, however, the thickness of the thin metal sheet of the color cathode-ray tube is smaller than 0.08 mm, particularly 0.05 mm as disclosed in Japanese laid-open patent publication No. 4-126341, then some difficulty arises as to the handling of the thin metal sheet itself.
More specifically, when slots or apertures are defined in the thin metal sheet, or when the thin metal sheet with the slots or apertures defined therein is inspected for any defects, or when the inspected thin metal sheet is welded to the frame, since the thin metal sheet is very thin and has the slots or apertures defined therein, with the web portions being joined only at the opposite ends of the thin metal sheet and hence soft and unstable, the web portions tend to be flexed, bent, and intertwined, causing wrinkles in themselves or joined ends thereof.
Once such wrinkles are produced, they are liable to remain in the thin metal sheet even when the thin metal sheet is subsequently mounted on the frame and kept taut under tension on the frame. Even if the thin metal sheet itself is found not defective, the eventual color selecting mask with the wrinkles is unable to land the electron beam accurately on desired color phosphors on the color phosphor screen. The color cathode-ray tube thus produced is defective and cannot be shipped from the factory. Accordingly, the yield of proposed color cathode-ray tubes is low.
In view of the fact that large-size color cathode-ray tubes for high-definition television (HDTV) are finding widespread use, thin metal sheets for use as aperture grilles in large-size and high-quality color cathode-ray tubes pose similar handling problems even when the thickness of the thin metal sheets is greater than 0.08 mm.